This invention relates to a continuous process for producing epoxy resin having a low content of easily hydrolyzable non-ionically bound chlorine (EHC) from chlorine-substituted epoxy resin obtained by epoxidation and polymerization from epoxy resin-forming chlorine-substituted starting material. In other aspects, the invention relates to apparatus for automatically controlling such novel process, and a novel measuring cell for use therein.
In the known process for the production of epoxy resins, the final products of the epoxidation and polymerisation usually have a content of EHC which is so high that it imparts to the resin an undesirable electrical conductivity. This is particularly disadvantageous when the epoxy resin is to be used in anti-corrosion coatings, e.g. as cataphoresis resins, or in the electrical and the electronic field. More particularly, electrical conductivity is practically unacceptable in boards for printed circuits, where it would severely reduce the range of applicability of epoxy resin as board material.
The crude resin which is obtained as a principal product in the production of epoxy resin contains about 0.2 to 1.5% by weight of EHC. The EHC-content consists preponderantly of 1,2-chlorhydrines of the formula ##STR1## in which R is an aliphatic or preferably an aromatic radical. The crude resin also contains certain amounts of unsaponifiable chlorine and ionic chlorine, i.e. chloride anions in a crystal lattice.
The term "chlorine" in this description and in the appended claims always means chlorine chemically bound in some manner and never chlorine gas.
The term "unsaponifiable chlorine" is used hereinafter and in the appended claims to mean chlorine atoms bound in an organic aromatic or aliphatic compound, in a manner such that they will not be saponified or "hydrolyzed" by the dehydrochlorinating agents and under the reaction conditions which are conventional in the production of epoxy resins. An example of unsaponifiable bound chlorine is shown in the following formula ##STR2## wherein R represents a preferably aromatic organic radical and R' represents preferably lower alkyl or hydrogen.
In order to obtain epoxy resin of lesser EHC-content, i.e., of 0.07% by weight or less, the crude resin is subjected to a dehydrochlorination treatment. Basic agents, in particular alkali metal and earth metal hydroxides such as sodium hydroxide, potassium hydroxide, lithium hydroxide or calcium hydroxide, alkali metal salts of weak acids such as potassium carbonate, potassium hydrogen carbonate, sodium methylate, potassium n-butylate or potassium ter-butylate, or quaternary ammonium hydrogen carbonates of the formula EQU (R.sub.4 N).sup.+ (HCO.sub.3).sup.-
in which R designates a preferably lower alkyl radical, are conventionally used as dehydrochlorinating agents.
By the dehydrochlorination treatment, a certain portion of EHC will be removed from the crude resin with formation of salts such as sodium chloride, potassium chloride, lithium chloride, calcium chloride, quaternary ammonium chloride and the like.
However, the exact dosing of the dehydrochlorinating agent during a continuous treatment is very critical and not easy to achieve.
For, in the equation ##STR3## there prevails an equilibrium which prevents the obtainment of an EHC-free product. Adding an excess of dehydrochlorinating agent (NaOH) leads to the undesirable formation of polymers which will disturb subsequent treatment steps such as evaporation of liquid and filtration of the end product.
Even the smallest excess amount of dehydrochlorinating agent will initiate polymerizing reactions in the resin. Moreover, polymerization products which are thus formed will gradually block lines for the flow of product, and cause failures in the filtering equipment of the plant; moreover, they will also deteriorate the quality of the finally obtained epoxy resin.
Therefore, the rate of adding sodium hydroxide or other dehydrochlorinating agent is very critical and should be strictly and accurately controlled.
Attempts have been made in the past to overcome this drawback by carrying out at frequent intervals, e.g. every 90 to 120 minutes, manual chemical "wet state" analyses to determine the amount of easily hydrolyzable chlorine in the product being continuously fed into the dehydrochlorination treatment. It is a serious drawback of this wet state analysis that numerous samples have to be taken and that a plurality of operations by hand have to be carried out in order to prepare each sample for testing, involving e.g. saponification and titration, so that the entire process becomes cumbersome and requires a great number of personnel.
A more serious drawback is caused by the fact that analysis results are received after a relatively long delay so that an adjustment of the feed rate of dehydrochlorinating agent can take place only rather late after a change in the EHC-content occurred while, in the meantime, a deficiency or excess of dehydrochlorinating agent has had its detrimental effect on the product. Moreover, the results of the above-mentioned method of analysis are relatively inaccurate.
Therefore, in order to avoid as completely as possible the occurrence of polymerization processes during the dehydrochlorinating treatment, a residual EHC-content of about 500 to as much as 700 ppm (0.05 to 0.07% by weight) is still deemed acceptable in epoxy resin produced by the conventional continuous process. Residual EHC-contents of 200 ppm or even less can be attained by the conventional continuous process only with additional costly EHC-removing treatments. In the unsatisfactory conventional batch process which is above all time-consuming and requires an excessive number of personnel, it would indeed be possible to produce epoxy resin having an EHC-content as low as 70 ppm but such resin would not be commercially acceptable, e.g., for electronic circuit boards, because of its high production costs.
Thus, a process for reducing the content of hydrolyzable chlorine in glycidyl compounds described in European patent application Publication No. 185,622 A2 by Ciba-Geigy AG, published on June 25, 1986 requires the use of considerable amounts of cellulose as an extraneous material in the system.
While it is therefore necessary to produce the epoxy resins usable in the electronic field by a continuous process, the product obtained thereby fails quite frequently to meet the specifications imposed by the customer.